Recording systems for visualizing image information through an electrostatic latent image, such as electrophotography, are now widespread in various fields. In electrophotography, an electrophotographic photoreceptor is charged and then exposed to light to form an electrostatic latent image, the latent image is developed with a developer containing a toner, and the toner image is transferred and fixed. The developer used herein includes a two-component developer comprising a toner and a carrier and a one-component developer comprising a toner, e.g., a magnetic toner, alone.
In the two-component developer, since a carrier bears such functions as agitation, delivery and charging of the developer, the two-component developer is characterized by satisfactory controllability and largely employed for the present time. In particular, developers using a resin-coated carrier are excellent in charge controllability and are relatively easy to improve environmental dependence and stability with time.
Development was formerly carried out by cascade development, but nowadays magnetic brush development using a magnetic roll as a developer carrier is dominant.
Magnetic brush development using a two-component developer has such disadvantages as reduction of image density and considerable background stain both due to reduction in charging properties of the developer, image roughness and consumption loss of the carrier both due to adhesion of the carrier onto the image, and occurrence of unevenness of image density. It is considered that with a reduction in resistance of the carrier, the induced charges are injected into the image area, resulting in adhesion of the carrier to the image area; or it is considered that charge quantity of the carrier after development becomes excessive on account of insufficient control of the upper limit of charge quantity of the carrier, resulting in adhesion of the carrier to the edges of the image area.
In recent years, negatively chargeable organic photoreceptors have been spread, and reversal development in which an electrostatic latent image is formed by using a laser, etc. has been frequently applied to inorganic photoreceptors. Accordingly, there have been increasing demands for high quality developers using a positively chargeable toner as well as a negatively chargeable toner. To this effect, it is desirable that the carrier to be used has freely controllable charging properties in accordance with the polarity and intensity of charge of the toner.
Fluorine-containing resin-coated carriers have been proposed as carriers for positively chargeable toners. However, fluorine-containing resins such as polyvinylidene fluoride have poor adhesion to core particles and often fail to retain charging properties in long-term use. Further, the resin coat is apt to fall off to reduce electrical resistance of the carrier, causing adhesion of the carrier to image areas, black spots due to the released coating material, and unevenness of image density. It has been proposed to use an adhesion-imparting resin in combination for improving adhesion of negatively chargeable resins onto core particles as disclosed in JP-A-54-110839 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), but sufficient effects cannot always be obtained.
High temperature treatment at 200.degree. C. or higher has been adopted for improving adhesion of a coating material to core particles. However, exposure of a resin containing fluorine in high concentrations to such high temperatures is unavoidably accompanied with evolution of harmful gases such as hydrogen fluoride, which possibly causes reduction in durability of equipment, deterioration of safety and hygiene, and environmental disruption. Although it is not impossible to lower the melting point of a fluorine-containing resin by, for example, copolymerization, a drop in melting point of a coating material tends to lessen the strength of the coat and to cause reduction in retention of charging properties.
It is also known to use a solution of a fluorine-containing resin to improve adhesion to core particles (solution coating method). However, the solvents used, e.g., dimethylformamide, have a high boiling point and high toxicity. Use of solvents of relatively low toxicity, such as methyl ethyl ketone and methyl isobutyl ketone, has been proposed, but fluorine-containing resins have insufficient solubility in these solvents so that coating must be conducted over a long period of time using a large quantity of the solvent.
Further, in conventionally employed solution coating methods using a fluidized bed apparatus or a spray drier, the coated carrier particles easily agglomerate with each other irrespective of the kind of the coating resin. It is therefore necessary to conduct deagglomeration treatment or to limit the amount of the resin to be coated, thus leaving problems of production process and product quality.
Since fluorine-containing resins generally have poor solubility in organic solvents, various solventless coating methods have been proposed. For example, it has been proposed to heat a powder mixture of core particles and a coating resin above a melting point of the coating resin in a still state to form a coating layer as disclosed in JP-A-54-35735. In this method, since formation of the coating layer is largely dependent on melt rheological behavior of the coating resin, it is very difficult to use a resin having a high viscosity in a molten state. Further, being coated in a still state, the carrier particles unavoidably agglomerate and require a post-treatment, such as deagglomeration treatment, thus attaining a poor production efficiency. Moreover, it is difficult to form a continuous coating layer having a smooth surface.
It has also been proposed to dry blend core particles and coating resin particles and heat-melt the blend in a fluidized bath or a rotary furnace, followed by cooling as disclosed in JP-A-55-118047, JP-A-60-170865 and JP-A-62-106475. However, since mixing of the core particles and coating resin particles depends on gravity shearing, the shearing force acting on particles is insufficient for thorough mixing, failing to form a uniform coating layer on the carrier particles. Particularly in cases where the core particles have a small size or a small specific gravity, or where the core particles are irregular in shape, or where the amount of the coating resin is large, the same problems as encountered in the above-described coating method of JP-A-54-35735 are involved.
Further, a coating method in which an impact force is repeatedly given to a mixture of core particles and coating resin particles to spread the coating resin to thereby form a coating layer has been proposed as disclosed in JP-A-63-235963, JP-A-63-235964, and JP-A-63-298254. However, the core particles are liable to be broken by the impact force. In addition, a controllable range of the coat thickness is narrowly restricted, and the coat thickness is less controllable than the solution coating method. As a result, this method is poor in controllability of resistance and charging properties.
In order to improve adhesion of negatively chargeable resins to core particles, it has been proposed to provide an intermediate layer therebetween as disclosed in JP-A-49-51950 or to blend a fluorine-containing resin with a second resin having strong adhesiveness to core particles as disclosed in JP-A-54-110839 and JP-A-56-113146. However, since the resins proposed exhibit opposite polarity to fluorine-containing resins, a blending ratio satisfying adhesion requirements consistently with charging properties is so limited. Although methyl methacrylate copolymers, etc. as proposed are excellent in compatibility with fluorine-containing resins and effective to improve adhesion, they adversely affect the excellent properties inherent in fluorine-containing resins such as solid lubricating properties and low tackiness, resulting in interference with improvement of surface stain resistance of the carrier.